a. Field of Invention
The invention relates to a method of controlling a-c power by means of thyristors, which is applied when a resistance-type electric furnace equipped with a transformer for supplying electric power conforming to the resistance heating elements in the furnace is operated and an apparatus for carrying out of the method.
In the field of electrical heating such as an electric furnace, temperature control is essential. In particular, a temperature control of high precision has been demanded recently in order to meet the requirement for quality control and to improve the yield of the furnace. In electric furnaces which employ resistance heating elements or infrared lamps as heat sources, almost all of the arrangements employ controlled rectifier elements such as thyristors or TRIAC's to effect an a-c power control. Most a-c power controls of the prior art which use thyristors achieve a power control by adjusting the firing phase angle.
b. Description of the Prior Art
As the a-c power control through the adjustment of the firing phase angle becomes popular, waveform distortions or equivalently a degradation in the power factor of the a-c power system is becoming a problem. To overcome this problem, a cycle control technique or zero crossing control technique in which the switching takes places at zero crossing of the source voltage is being increasingly applied. In the cycle control technique, a time interval of a suitable length, for example, a time interval corresponding to 120 cycles, is chosen as one control period, and the a-c power control is achieved by suitably changing the ratio of the on time and the off time within the control period (see FIG. 2). While the advantage of the cycle control technique is recognized, it has not gained enough popularity to be substituted for the firing angle control because of the following drawbacks:
(i) To provide an output accuracy of 1%, a control period of about two seconds corresponding to 100 cycle time is required. When detecting an effective voltage, current or power at the controlled output, ripples having about two second cycles must be removed, with resulting difficulty in improving the accuracy of detection. The lag in the detection also increases. PA0 (ii) To change the parameter being controlled during one control period, it is necessary to wait for the next control period to commence before an output can be actually changed, resulting in an increased dead time between a change in the output relative to a change in the input. PA0 (iii) If the length of the control period is reduced to overcome the above disadvantages, there results a reduction in the output accuracy, preventing the desired accuracy of temperature from being attained.